RE: [PATCH v5 00/15] x86: Add support for Clang CFI

David Laight
2021-10-27T10:03:00Z

From: Peter Zijlstra
> Sent: 26 October 2021 21:16
>
> On Wed, Oct 13, 2021 at 11:16:43AM -0700, Sami Tolvanen wrote:
> > This series adds support for Clang's Control-Flow Integrity (CFI)
> > checking to x86_64. With CFI, the compiler injects a runtime
> > check before each indirect function call to ensure the target is
> > a valid function with the correct static type. This restricts
> > possible call targets and makes it more difficult for an attacker
> > to exploit bugs that allow the modification of stored function
> > pointers. For more details, see:
> >
> > https://clang.llvm.org/docs/ControlFlowIntegrity.html
>
> So, if I understand this right, the compiler emits, for every function
> two things: 1) the actual funcion and 2) a jump-table entry.
>
> Then, every time the address of a function is taken, 2) is given instead
> of the expected 1), right?
>
> But how does this work with things like static_call(), which we give a
> function address (now a jump-table entry) and use that to write direct
> call instructions?
>
> Should not this jump-table thingy get converted to an actual function
> address somewhere around arch_static_call_transform() ? This also seems
> relevant for arm64 (which already has CLANG_CFI supported) given:
>
> https://lkml.kernel.org/r/20211025122102.46089-3-frederic@kernel.org
>
> Or am I still not understanding this CFI thing?

From what I remember the compiler adds code prior to every jump indirect
to check that the function address is in the list of valid functions
(with a suitable prototype - or some similar check).

So it add a run-time search to every indirect call.

What would be more sensible would be a hidden extra parameter that is
a hash of the prototype that is saved just before the entry point.
Then the caller and called function could both check.
That is still a moderate cost for an indirect call.

Anything that can write asm can get around any check - it just gets a
bit harder.
But overwritten function pointers would be detected - which I assume
is the main threat.

David

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